function remSize() {
  let deviceWidth = document.documentElement.clientWidth
      || window.innerHeight;
  if (deviceWidth >= 750) {
    deviceWidth = 750;
  }
  if (deviceWidth <= 320) {
    deviceWidth = 320;
  }
  document.documentElement.style.fontSize = deviceWidth / 7.5 + 'px';
  document.querySelector('body').style.fontSize = 0.3 + 'rem';
}

remSize();

window.onresize = function() {
  remSize();
};